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[PATCH] Don't decrease mem_cost for special MEMs, even with REG_EQUIV (PR rtl-optimization/52208)


As mentioned in the PR, if SET_SRC MEM (with REG_EQUIV note) isn't a MEM
that can be used in general operations, but requires some specialized
instruction to load it, decreasing its mem_cost results sometimes in
worse code, the MEM is loaded with a specialized insn, then stored into
a stack slot because e.g. a negative memory cost has been chosen as
cheapest, then loaded back from the stack slot.

This patch gives up if the MEM isn't general_operand.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2012-02-16  Jakub Jelinek  <>

	PR rtl-optimization/52208
	* ira-costs.c (scan_one_insn): Don't decrease mem_cost
	for MEMs with REG_EQUIV, if the MEM isn't general_operand.

--- gcc/ira-costs.c.jj	2012-02-15 07:34:54.000000000 +0100
+++ gcc/ira-costs.c	2012-02-15 20:57:49.239393641 +0100
@@ -1306,14 +1306,21 @@ scan_one_insn (rtx insn)
      Similarly if we're loading other constants from memory (constant
      pool, TOC references, small data areas, etc) and this is the only
-     assignment to the destination pseudo.  */
+     assignment to the destination pseudo.
+     Don't do this if SET_SRC (set) isn't a general operand, if it is
+     a memory requiring special instructions to load it, decreasing
+     mem_cost might result in it being loaded using the specialized
+     instruction into a register, then stored into stack and loaded
+     again from the stack.  See PR52208.  */
   if (set != 0 && REG_P (SET_DEST (set)) && MEM_P (SET_SRC (set))
       && (note = find_reg_note (insn, REG_EQUIV, NULL_RTX)) != NULL_RTX
       && ((MEM_P (XEXP (note, 0)))
 	  || (CONSTANT_P (XEXP (note, 0))
 	      && targetm.legitimate_constant_p (GET_MODE (SET_DEST (set)),
 						XEXP (note, 0))
-	      && REG_N_SETS (REGNO (SET_DEST (set))) == 1)))
+	      && REG_N_SETS (REGNO (SET_DEST (set))) == 1))
+      && general_operand (SET_SRC (set), GET_MODE (SET_SRC (set))))
       enum reg_class cl = GENERAL_REGS;
       rtx reg = SET_DEST (set);


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